In science fiction, aliens have come to Earth to seize the planet's resources, conduct scientific experiments, or, in the case of ALF, because their home world was destroyed. But experts say that would be illogical assume that real-life extraterrestrials would come to our planet for these reasons.

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The human race could be devastated if aliens were to learn of our existence and venture to Earth, warned British scientist Stephen Hawking on Sunday. But how could extraterrestrials really invade Earth?

Aliens have already viciously attacked our spacecraft, savagely kidnapped us, heartlessly conducted experiments on us, and mercilessly aimed their death-rays at us, but of course, all of these crimes have been committed only in novels and movies.

"In movies, aliens only come here for two reasons," Seth Shostak, senior astronomer at the Search for Extraterrestrial Intelligence (SETI) told Life's Little Mysteries, SPACE.com's sister publication. "They either come here to find some resource they don't have on their own planet, or they want to use us for some unauthorized breeding experiment." These scenarios play on our most primal human fears of losing the resources we need to survive or not being able to reproduce, Shostak said.

In reality, it isn't logical to think that aliens would want to do either of those things, Shostak said. Space travel is expensive and requires an enormous investment, he said.

"Anything that we have here, they could find where they live," Shostak said. If there was a resource found on Earth that did not exist on the aliens' home planet, there would certainly be easier ways to get or make the resource than coming here.

And if an alien civilization was advanced enough to engage in interstellar travel, they would also probably have very advanced robotic machines, Shostak said. If they wanted to research our planet, they would be more likely to send those machines here than to come here themselves.

"It's not like, the hatch will open and we'll see a strange, alien paw coming out," he said. "It's more likely to be a robotic arm."

Contact with aliens is extremely unlikely, agrees David Morrison, Director of Space at NASA-Ames Research Center. Any communication that may occur would likely be in the form of radio waves sent from one civilization to another, he said.

"We're listening for radio signals," Morrison said, "And we can assume that any civilization that we receive a signal from is more advanced than we are."

We have only had the technology to listen and send radio waves for the last century, so if an alien radio signal reaches us from a distant planet hundreds or thousands of light-years away, that civilization would have to be more advanced than ours, Morrison said.

Morrison doubts that an advanced alien civilization would come here to harm us.

"Someone once suggested that if a civilization can last for hundreds of thousands of years, it almost surely has solved the problems we have. I would hope so," Morrison said.

Even if aliens existed, knew about us, and could travel here, they wouldn't be likely to send an army or the equipment needed to launch an attack on the Earth, said science fiction writer Jack McDevitt.

"Imagine putting together an invasion force, only to stick them in containers to travel here for years," McDevitt said.

Although contact between humans and aliens has been a key part of many of McDevitt's books, he doesn't think that it's likely to actually happen. It would take a great amount of time for aliens to reach Earth, and any civilization capable of this feat would not want to delegate its fighting force to the task, he said.